1. Field of the Invention
The present invention relates to a flow rate control apparatus capable of controlling the flow rate of a fluid flowing through a fluid passage. In particular, the present invention relates to a flow rate control apparatus capable of reducing pressure loss occurring in the pressure fluid flowing through the fluid passage.
2. Description of the Related Art
A flow rate control apparatus has been hitherto used to control the flow rate of a pressure fluid flowing through a fluid passage. In such a flow rate control apparatus, the rotary shaft of a rotary driving source, such as a stepping motor, is rotated so that a displacement member, which is screw-engaged with the rotary shaft, is displaced in an axial direction. A valve plug, which is connected to the displacement member, is seated on and separated from a valve seat, under a displacement action of the displacement member, in order to control the flow rate of the pressure fluid flowing through the fluid passage.
For example, a conventional technique used in connection with a flow rate control apparatus is shown in FIG. 4, and adopts the following arrangement. That is, a rotor 2, which is arranged in a stepping motor 1, is rotated under a driving action of the stepping motor 1, which serves as a driving source. A driving rod 3, which is screw-engaged with a central portion of the rotor 2, is moved back and forth in an axial direction under the rotary action of the rotor 2 (see Japanese Laid-Open Patent Publication No. 11-136925).
A valve plug 4, which is connected to a forward end of the driving rod 3, is displaced integrally under the displacement action of the driving rod 3, and the valve plug 4 is separated from an outflow port 6 of a valve body 5. Accordingly, fluid, which is introduced from an inflow port 7 of the valve body 5, flows to the outside via an outflow port 6.
A thin-walled diaphragm 8, which is flexibly deformable, has an inner circumferential portion, which is interposed between the valve plug 4 and the forward end of the driving rod 3. An outer edge portion of the diaphragm 8 is interposed between portions of the valve body 5. The diaphragm 8 retains air tightness with respect the fluid flowing through the interior of the valve body 5.
In the case of the flow rate control valve according to Japanese Laid-Open Patent Publication No. 11-136925, the degree to which the valve plug 4 is opened changes occasionally, depending on conditions of use thereof, which act to increase or decrease the flow rate of fluid flowing through the interior of the valve body 5.
For example, when the flow rate of fluid flowing through the interior of the valve body 5 is increased or decreased, the valve plug 4 and diaphragm 8 are displaced in a direction to open the valve (i.e., in the direction of arrow X1) or in a direction to close the valve (i.e., in the direction of arrow X2). During this process, the fluid pressure of the fluid is applied to the valve plug 4 and diaphragm 8, which are displaced in a direction to open the valve, and resistance is generated in the flow of fluid as a result of the valve plug 4 and diaphragm 8. Therefore, a pressure loss appears in the fluid.
When the valve opening degree of the valve plug 4 is decreased, under a driving action of the stepping motor 1, it is necessary for the valve plug 4 to be displaced in a direction to close the valve (i.e., in the direction of arrow X2) against the direction of the flow of fluid that flows through the interior of the valve body 5. Therefore, a large load is exerted on the stepping motor 1, as compared to the process in which the valve opening degree of the valve plug 4 is increased.
As a result, it is necessary to use a stepping motor 1 which has a large driving force sufficient to displace the valve plug 4 and diaphragm 8 against the fluid pressure of the fluid. Therefore, a large-scale stepping motor 1 has been required.